This invention relates to a method of producing a photoanode for use in a photoelectrochemical (PEC) cell.
In its simplest form, a PEC cell consists of two electrodes immersed in an aqueous electrolyte and connected electrically by a wire. One of these electrodes is a metal that does not react chemically with the electrolyte; the other electrode is a semiconductor with one face in contact with the electrolyte and the other face connected to the shorting wire by an ohmic contact. Ideally, when light falls on the semiconductor electrode, oxygen gas is liberated at one electrode and hydrogen is liberated at the other.
The operation of such a cell can be generally explained in terms of electron energy levels in the electrodes and the electrolyte. For an n-type semiconductor photoanode, light incident upon the semiconductor with energy hv greater than the energy gap of the material, E.sub.g, results in the generation of an electron-hole pair. This pair is separated by the electric field in the depletion region. Under the influence of this electric field the electrons move away from the surface into the bulk of the semiconductor and then transfer via the external circuit to the metal counter-electrode where they discharge H.sub.2 according to the reaction: EQU 2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 .uparw.(Cathode)
The holes, on the other hand, move to the semiconductor-electrolyte interface and discharge O.sub.2 according to the oxidation reaction: EQU OH.sup.- +2p.fwdarw.1/2O.sub.2 .uparw.+H.sup.+ (Photoanode)
For p-type semiconducting photoanodes, a hole depletion region is formed with the photogenerated electrons moving to the semiconductor-electrolyte interface and the holes transferred via the external circuit to the metal counter-electrode (anode). Accordingly, hydrogen is liberated at the semiconductor electrode and oxygen at the metal counter-electrode.
Titanium dioxide (TiO.sub.2) is one of the most commonly used material for making photoanodes in PEC cells. TiO.sub.2 based photoanodes have been fabricated using single crystals of TiO.sub.2 and polycrystalline TiO.sub.2 prepared by a variety of techniques including chemical vapor depositions, thermal oxidation or anodic oxidation of thin Ti foils, hot pressing of TiO.sub.2 powder, and plasma spraying.
TiO.sub.2 is an n-type semiconductor with high electrical resistivity. Conductivity in TiO.sub.2 is induced by heating in a reducing atmosphere which results in the formation of TiO.sub.2-x, where x takes on a value between 0 and 1. The most common reducing atmosphere is hydrogen, although reduction in vacuum, argon, nitrogen, forming gas and CO/CO.sub.2 mixture is also known to the art.
TiO.sub.2 thick film dielectric materials are well established in the electronic industry for the fabrication of multilayer capacitors. For this application, the TiO.sub.2 thick film contains a high percentage of glass component, varying between 10% and 90%, to provide cohesion of the TiO.sub.2 particles and adhesion of these particles to the substrate. In this case, the as-fired film consists of TiO.sub.2 particles embedded in a glassy matrix and exhibits high dielectric constant as well as high breakdown voltage.
The presence of a glass frit binder in the TiO.sub.2 is an undesirable feature for its use as a photoanode in PEC cells. In this application, the glass component would reduce the active region for the absorption of light in the photoanode and decrease the efficiency of electron transfer at the electrolyte-semiconductor interface. Also, it increases the resistivity of the film which leads to higher ohmic losses in the PEC cells.
U.S. Pat. No. 4,090,933 of Nozik, issued May 23, 1978, describes the basic aspects of a complete photoelectrochemical cell, i.e. the general properties and geometrical configurations of all of the elements of such a cell. The patent does not, however, address the preparation of the electrodes except for the use of TiO.sub.2 in the form of a single crystal or RF sputtered film.
Two other patents relating to similar subject matter are U.S. Pat. Nos. 4,181,593 of McKinzie et al, issued Jan. 1, 1980 and 4,215,155 of McKinzie et al, issued July 29, 1980. These patents disclose the use of undoped and doped polycrystalline n-type TiO.sub.2 as photoanodes in photoelectrochemical cells. The anode preparation procedure generally consists of applying a slurry of a mixture of TiO.sub.2 powder, and other oxides where applicable, with a suitable consistency to paint onto a metal substrate, heating in an oxygen-containing atmosphere at 500.degree.-800.degree. C., and reducing in hydrogen at elevated temperatures. [The present process, however, uses screen printing technology to obtain precisely defined deposition parameters. The same technology may also be used for applying a metal backing onto a ceramic substrate. Also, in the adhesive mechanism used to adhere TiO.sub.2 to the substrate, specific use is made of properties associated with a phase transition (from anatase to rutile) in TiO.sub.2 at elevated temperatures. Thus, the process according to the invention is quite different from those described in these two patents.]
U.S. Pat. No. 4,216,071 of Gobrecht, issued Aug. 5, 1980 describes a cell for electrode deposition in which anodes consist of a metallized ceramic or a metal support covered by a layer of semiconducting material. Various methods of applying the semiconductor layer are mentioned, such as vapour deposition, cathodic sputtering, or coating from a liquid phase. There is, however, no disclosure of the method according to the invention which utilizes screen printing.